Laser ablatable material and its use

ABSTRACT

An epoxy based resin which exhibits good laser ablation and good adherence to a substrate such a copper is provided by adding to the resin a dye or dyes having substantial energy absorption at the emission wave lengths of lasers used to laser ablate the resin. The resin with the dye or dyes included is coated onto a substrate and cured, or laminated onto a substrate in the cured condition. The required openings are formed in the cured film by laser ablation. This allows for the use of optimum techniques to be used to form micro vias.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of application Ser. No. 09/375,957,filed Aug. 17, 1999 U.S. Pat. No. 6,361,923.

FIELD OF THE INVENTION

This invention relates generally to a laser ablatable material and itsuse and, more particularly, to an epoxy base laser ablatable materialand its use to form a dielectric substrate having micro vias therein

BACKGROUND INFORMATION

One typical technique for forming a dielectric layer of material withvias therein is to utilize an epoxy base, photoimageable material coatedonto a substrate, such as another dielectric material or a conductivematerial, and form the vias or other openings therein byphotolithographic techniques. Particularly suitable materials aredescribed in U.S. Pat. No. 5,026,624 and 5,300,402, and U.S. patentapplication Ser. No. 09/212,204, filed Dec. 15, 1998, all commonlyassigned, and all of which are incorporated herein by reference. Asdescribed in these patents and patent application, the photoimageablematerial is coated onto a substrate and U.V. light of the appropriatewavelength is used to expose the desired pattern on the photoimageablematerial, after which the material is developed to form the desired viasand other openings in the material.

For many applications, this is a very successful technique. However, inspecific applications, certain difficulties may be encountered in usingphotolithographic techniques. For example, in certain applications,copper foil is laminated to a dry film of material made according to theteachings of either U.S. Pat. No. 5,665,650 or 5,670,750 and results invery narrow process windows and manufacturing limitations. Inparticular, to achieve optimum adhesion of the foil to the dielectricmaterial, a low exposure dose is required. However, too low an exposurewill induce defects upon stressing, and too high an exposure dose willresult in degradation of the bond between the foil and the dielectricmaterial. Thus, in some cases, an optimum dosage to achieve good peelstrengths is not adequate for small micro via definition. Moreover, whenexposure through artwork is required, defects, such as dust particles,scratches etc., will print through, leaving a defect in the dielectriclayer. With laser ablation, photo induced defects will not appear on thedielectric layer.

SUMMARY OF THE INVENTION

According to the present invention, an epoxy based resin which exhibitsgood laser ablation and good adherence to a substrate, such as copper,is provided by adding to the resin a dye or dyes having substantialenergy absorption at the emission wave lengths of lasers used to laserablate the resin. In one embodiment, the resin is coated onto asubstrate and cured, or laminated onto a substrate. Then the cured filmis laser ablated to form the desired pattern of openings. The film canalso be either laminated or coated and cured onto a copper foil, and thedielectric with the copper foil thereon laminated to a substrate, Thisallows for the use of optimum techniques to be used to form micro vias.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to the present invention, an improved epoxy based laserablatable material is provided. The improvement in laser ablatability isachieved by incorporating into the epoxy based composition one or moredyes which increase the energy absorption of the base composition at theemitted wave length of the laser that is used to laser ablate. As iswell known in the art, different lasers emit at different wavelengths.Some examples include excimer lasers which emit at either 308 nm or 248nm, YAG lasers which emit at 532 nm, 355 nm, or 266 nm, and CO₂ laserswhich emit at 10600 nm. Thus, a dye that has an absorbance at or nearthese wavelengths incorporated into the epoxy base will significantlyenhance the laser ablation properties of the epoxy based resin. Forexample, Rhodamine B has a maximum absorbance at 543 nm, Ethyl Violethas a maximum absorbance at 596 nm, 2′, 7′dichlorofluorescein has amaximum absorbance at 512 nm, and fluorescein has a maximum absorbanceat 498 nm, all of which dyes exhibit significant absorbance at 532 nmand, thus, are useful dyes for a YAG laser emitting at 532 nm. Dyeswhich exhibit significant absorbency at the 355 nm wavelength arecarbostyril 124 (maximum absorbence 349 nm) carbostyril 165 (maximumabsorbence 360 nm) coumarin 2 (maximum absorbence at 365 nm) andcoumarin 120 (maximum absorbency 352 nm) and, thus, are useful forincorporation into epoxy based resins to be ablated by YAG lasers thatemit at 355 nm wave length. Coumarin 4 (maximum absorbency 322 nm) isuseful for lasers emitting at 308 nm and p-terphenyl (maximum absorbency276 nm) is useful for lasers emitting at 266 nm.

One or more of the energy absorbing dyes described above are added tothe epoxy based resin, preferably at about 0.01 to about 1.0 part byweight per 100 parts by weight of the epoxy resin. A single dye can beutilized, or multiple dyes can be used to increase the ablatability tolasers emitting at different wavelengths.

The material may also be photoimageable by photolithographic techniques,in which case a photoinitiator, preferably triarylsulfoniumhexafluroantimonate, is included in the composition. Other usefulphotoinitiators include diphenyl iodonium salts and their derivatives. Aparticular advantage to Rhodamine B and Ethyl Violet is that when theyare used with compositions that contain photoinitiators, such astriarylsulfonium hexafluroantimonate, they do not interfere with thephotochemical and curing reactions.

Alternatively, thermally labile cationic photoinitiators that initiatepolymerization of the epoxy functionality by exposure to light or byexposure to temperatures above about 125° C., such as alkoxy substitutedaryl onium salts of the type disclosed in U.S. Pat. No. 4,882,221 and5,079,778, can be utilized. On the other hand, the addition of anthroneor other curing agents will allow epoxy formulations to be thermallystable up to about 150° C., above which temperature it will polymerize.Other curing agents can be chosen to cure the film as low as 80° C. Theepoxy based composition with the dye or dyes added thereto is especiallyadapted for use as a dielectric material to coat onto a substrate toform a circuitized I/C chip carrier or circuit board with micro viasformed therein. The material can be applied as a solution and thendried, as disclosed in U.S. Pat. No. 5,026,626, and 5,330,402, or it canbe formed into a dry film and applied to the substrate by laminatingtechniques as disclosed in U.S. patent application Ser. No. 09/212,204.Also, a solventless film can be formed and applied to a substrate, suchas a copper foil, and cured.

There are several techniques for applying and further processingdielectric film on substrates. One technique for applying film is tofirst apply the film to a substrate in either dry film form or coatedfilm form and cured. Vias are then laser ablated, following which copperis plated onto the surface and into the vias and then patterned. Inanother technique, after the dielectric film has been applied orlaminated, a copper foil is laminated onto the surface of the dielectricfilm and the film cured before the vias are formed. Vias are then laserablated through both the copper foil and the dielectric film, afterwhich the vias are plated with copper and the copper foil is patterned.Alternatively, the copper can be etched at the locations where the viasare to be formed, and then the vias laser ablated. In yet anothertechnique, the dielectric film can be either laminated to a copper foil,or coated onto a copper foil. The dielectric film, with the copper foilbacking is then laminated to the substrate, cured, and processed as inthe previous example. In any event, the laser ablation technique lendsitself to forming circuitized substrates.

Solvent Containing Dielectric Film

One especially useful epoxy based polymeric dielectric has solids whichare preferably comprised of from about 10% to 80%, preferably from 20%to 40%, more preferably about 30%, of phenoxy polyol resin which is thecondensation product of epichlorohydrin and bisphenol A, having amolecular weight of from about 40,000 to 130,000, preferably about60,000 to 90,000, more preferably greater than 60,000; from about 0% to80%, preferably from about 25% to 30%, most preferably about 25%, of anepoxidized multifunctional bisphenol A formaldehyde novalac resin havinga. molecular weight of from about 4,000 to 10,000, preferably about5,000 to 7,000; from 0% to 90%, preferably from about 35% to 50%, morepreferably 40% to 45%, most preferably about 45%, of a diglycidyl etherof bisphenol A, preferably brominated, having a molecular weight of fromabout 600 to 2,500, preferably about 1,000 to 1,700. A dye or dyes thatabsorb energy significantly at the emitting wavelength for the laserablation for one or more lasers is added to the epoxy base, preferablyat from about 0.01 to about 1.0 part by weight per 100 parts by weightof the resin component. If the composition is to be photoimageable, aphoto initiator is present at from about 0.1 to about 15 parts,preferably about 5 parts by weight, of the total resin weight.Preferably, the photoinitiator is a cationic photoinitiator havingsulfur moiety such as a complex triarylsulfonium hexafluoroantimonatesalt. A suitable complex triarysulfonium hexafluoroantimonate saltcationic photoinitiator formerly available as UVE 1014 from GeneralElectric Company, is now available as UVI 6974 from Union CarbideCompany. The solvent component of the dielectric film preferably iscomprised of propylene glycol monomethyl ether acetate, and 0% to lessthan about 10% propylene carbonate and 0% to less than about 50% ofmethyl ethyl ketone. The propylene carbonate is preferably the conveyorfor the preferred photoinitiator if a photoinitiator is to beincorporated in the composition.

Preferably, the phenoxy polyol resin has an epoxide value of from about0.001 to about 3, more preferably from about 0.01 to about 0.3, mostpreferably about 0.03 equivalents per kg, a weight per epoxide of fromabout 10,000 to about 60,000, more preferably from about 20,000 to about50,000, most preferably about 37,000 and a glass transition temperatureof from about 80 to about 150, more preferably from about 90 to about110, most preferably about 98° C.

Preferably, the multifunctional epoxy bisphenol A formaldehyde novolacresin has an epoxide value of from about 1 to about 10, more preferablyfrom about 3 to about 6, most preferably about 4.7 equivalents perkilogram, a weight per epoxide of from about 180 to about 300, morepreferably from about 190 to about 230, most preferably about 215 and amelting point of from about 60° C. to about 150° C., more preferablyfrom about 70° C. to about about 90° C., most preferably about 82° C.

Preferably, the diglycidyl ether of the bisphenol A has an epoxide valueof from about 0.1 to about 5, more preferably from about 1 to about 3,most preferably about 1.5 equivalents per kilogram, a weight per epoxideof from about 200 to about 1,000, more preferably from abut 500 to about750, most preferably about 675 and a melting point of from about 70 toabout 150, more preferably from about 80 to about 110, most preferablyabout 97° C.

A suitable phenoxy polyol resin is available under the trade name“PKHC”, formerly available from Union Carbide Corporation, and now fromPhenoxy Resin Associates. A suitable octafunctional bisphenol A,formerly available under the trade name “Epirez SU-8” from High TekPolymers, is now available as “Epon SU8” from Shell Chemical Company. Asuitable tetrabromobisphenol A, formerly available under the trade name“Epirez 5183” from High Tek Polymers, is now available as “Epon 1183”from Shell Chemical Company.

If a photoinitiator is not used so that there is not the photoimagingcharacteristic of the film, thermal curing agents, such asdicyanodiamide (DICY) or bisphenol or substituted bisphenols, can beadded. Catalytic agents, such as 2-methylimidazol or 2-ethyl,4-methylimidizol, can also be incorporated into the composition.

The solids of the photoimageable dielectric film optionally comprise aparticulate rheology modifier, preferably a thixotropic particulaterheology modifier. Preferably, the particulate rheology modifier has anaverage particle size of from about 0.001 to about 10 microns, morepreferably from about 0.01 to about 5 microns. Examples of particulaterheology modifiers are barium sulfate, talc, aluminum oxide, antimonyoxide, kaolin, finely divided silicon dioxide which may be colloidal orrendered hydrophobic, micronised talcum, micronised mica, kaolin,aluminum oxide, aluminum hydroxide, calcium silicate, aluminum silicate,magnesium carbonate, calcium carbonate, zirconium silicate, porcelainpowder, glass powder, antimony trioxide, titanium dioxide, bariumtitanate and barium sulfate or mixtures thereof Preferably, theparticulate rheology modifier is silica; a suitable silica is availableunder the trade name Aerosil A380 silica from Degussa. Where theparticulate rheology modifier is a thixotropic particulate rheologymodifier, it is preferred that no more than 30% be used.

Optionally, a surfactant, such as for example a nonionic surfactant, isemployed. Preferably, the surfactant is a fluorinated polyether; asuitable surfactant is available under the trade name FC-430 from 3MCompany In other embodiments, solventless epoxy systems are used. Apreferred solventless system is disclosed in commonly assigned U.S.patent application Ser. No. 09/212,204, filed 12/15/98, which isincorporated herein by reference. Again, a photoinitiator may be used ifphotoimaging is required, but can be omitted if photoimaging is notrequired.

The Solventless Dielectric Film

The solventless dielectric film is a high resolution dielectricmaterial. Preferably, the dielectric constant of the dielectric film isless than about 5, more preferably less than about 4. The dielectricfilm is thermally stable up to about 340° C. The dielectric filmpreferably is comprised of about 95% or more solids. The epoxy resinsystem comprises from about 10% to 80% of phenoxy polyol resin which isthe condensation product of epichlorohydrin and bisphenol A, having amolecular weight of from about 40,000 to about 130,000; from about 0% toabout 80% of an epoxidized multifunctional bisphenol A formaldehydenovolac resin, having a molecular weight of from about 4,000 to about10,000; from 10% to 50% of a diglycidyl ether of bisphenol A, having amolecular weight of from about 600 to 2,500; from about 10% to about 35%liquid epoxy resin, having a molecular weight of from about 200 to about600, preferably from about 250 to about 450. The “liquid epoxy resins”are liquid at 20° C. Preferably, the liquid epoxy resins are selectedfrom the group consisting of a cycloaliphatic epoxy resin, a bisphenol Aepoxy resin, and mixtures thereof

In one embodiment of the solventless film, the solids comprise an epoxyresin system which is preferably comprised of from about 10% to about80%, preferably from 10% to about 40%, more preferably from about 15% toabout 30% of the phenoxy polyol resin, which is the condensation productof epichlorohydrin and bisphenol A, having a molecular weight of fromabout 40,000 to about 130,000, preferably about 60,000 to about 90,000,more preferably greater than 60,000; from 0% to about 80%, morepreferably from about 12% to about 30%, most preferably from about 15%to about 20%, of an epoxidized multifunctional bisphenol A formaldehydenovolac resin, having a molecular weight of from about 4,000 to about10,000, preferably about 5,000 to about 7,000; from about 10% to about50%, preferably from about 25% to about 40%, more preferably about 27%to about 35% of a diglycidyl ether of bisphenol A, having a molecularweight of from about 600 to about 2,500, preferably about 1,000 to about1,700; from about 10% to about 35%, preferably from about 13% to about32%, more preferably from about 20% to about 30%, liquid epoxy resin. Ifthe film is to be photoimageable, from about 0.1 to about 15 parts,preferably about 5 parts by weight to 100 parts by weight of the totalresin weight, of a cationic photoinitiator is added. The solidsoptionally comprise a particulate rheology modifier from 0 to about 30%,preferably from 0.25% to about 30%, preferably from about 0.3% to about5%, most preferably from about 0.5% to about 4%.

In another embodiment of the solventless film, the solids comprise anepoxy resin system which is preferably comprised of from about 5% toabout 80%, preferably from 10% to about 40%, more preferably from about15% to about 30% of the phenoxy polyol resin, 0% of the epoxidizedmultifunctional bisphenol A formaldehyde novolac resin, from about 20%to about 80%, preferably from about 30% to about 70%, more preferablyabout 40% to about 60% of the diglycidyl ether of bisphenol A; fromabout 10% to about 35%, preferably from about 13% to about 32%, morepreferably about 20% to about 30%, liquid epoxy resin. If the film is tobe photoimageable from about 0.1 to about 15 parts, preferably about 5parts by weight, of the total resin weight, a cationic photoinitiator isadded. The solids optionally comprise a particulate rheology modifierfrom 0% to about 30%, preferably from 0.25% to about 30%, preferablyfrom about 0.3% to about 5%, most preferably from about 0.5% to about4%.

The diglycidyl ether of bisphenol A is preferably halogenated, morepreferably bromonated. The solvent component left over after processingof the photoimageable dielectric film preferably is comprised ofpropylene glycol monomethyl ether acetate, 0% to less than about 10% ofthe solvent, propylene carbonate, 0% to less than about 50% methyl ethylketone. The propylene carbonate is preferably the carrier for thepreferred photoinitiator.

Preferably, the phenoxy polyol resin has an epoxy value of from about0.001 to about 3, more preferably from about 0.01 to about 0.3, mostpreferably about 0.03 equivalents per kg, a weight per epoxide of fromabout 10,000 to about 60,000, more preferably from about 20,000 to about50,000, most preferably about 37,000 and a glass transition temperatureof from about 80° to about 150° more preferably from about 90° to about110°, most preferably about 98° C.

Preferably, the multifunctional epoxy bisphenol A formaldehyde novolacresin has an epoxy value of from about 1 to about 10, more preferablyfrom about 3 to about 6, most preferably about 4.7 equivalents perkilogram, a weight per epoxide of from about 180 to about 300, morepreferably from about 190 to about 230, most preferably about 215 and amelting point of from about 60° C. to about 150° C., more preferablyfrom about 70° C. to about 90° C., most preferably about 82° C.

Preferably, the diglycidyl ether of the bisphenol A has an epoxy valueof from about 0.1 to about 5, more preferably from about 1 to about 3,most preferably about 1.5 equivalents per kilogram, a weight per epoxideof from about 200 to about 1000, more preferably from about 500 to about750, most preferably about 675 and a melting point of from about 70° C.to about 150° C., more preferably from about 80° C. to about 110° C.,most preferably about 97° C.

The liquid epoxy resin has a weight average molecular weight of fromabout 200 to about 500, preferably from about 250 to about 450.Preferably, the liquid epoxy resin is either a bisphenol A epoxy resinor cycloaliphatic epoxy resin. The bisphenol A epoxy resin is a reactionproduct of bisphenol A and epichlorohydrin, and has an epoxy value offrom about 10 to about 4, more preferably from about 7 to about 5, mostpreferably about 5.5 equivalents per kilogram, a weight per epoxide offrom about 100 to about 250, more preferably from about 150 to about200, most preferably about 180, a weight average molecular weight offrom about 200 to about 500, preferably from about 250 to about 450,more preferably from about 300 to about 400, and a melting point ofbelow about 20° C. A suitable bisphenol A epoxy resin is a difunctionalbisphenol A epoxy resin available under the tradename Epon 826. The Epon826 resin from Shell Oil Corporation has an epoxide equivalent weight offrom about 178 to about 186 and a density of 1.6 grams/cm²

Preferably, the cycloaliphatic epoxy resin has an epoxy value of fromabout 10 to about 5, more preferably from about 8 to about 6, mostpreferably about 7.3 equivalents per kilogram, a weight per epoxide offrom about 100 to about 200, more preferably from about 120 to about150, most preferably about 137, a weight average molecular weight offrom about 200 to about 500, preferably from about 250 to about 450, andpreferably a melting point of below about 20° C. Preferably, thecycloaliphatic epoxy resin is a cycloaliphatic difunctional epoxy resin,more preferably cycloaliphatic epoxy resin is 3, 4-epoxycyclohexylmethyl3, 4-epoxycyclohexane-carboxylate. A suitable 3, 4-epoxycyclohexylmethyl3, 4-epoxy-cyclohexane-carboxylate resin is sold by Union Carbide underthe trademark “ERL-4221.” This resin has an epoxy equivalent weight offrom 131 to about 143, a freezing point of less than −20° C., a specificgravity of 1.18, and an approximate average molecular weight of fromabout 262 to about 286.

Other suitable liquid epoxy resins are vinyl cyclohexene dioxide,available under the. trade name “ERL-4206” from Union Carbide, 2-(3,4-epoxycyclohexyl-5, 5-spiro-3, 4-epoxy) cyclohexane-meta-dioxane,available under the trade name “ERL-4234” from Union Carbide, Bis (3,4-epoxy cyclohexyl) adipate, available under the trade name “ERL 4299”.ERL-4299 has a freezing point of approximately 9° C., a specific gravityof 1.15 and an epoxy equivalent weight of from about 190 to about 210and an average approximate molecular weight of from about 380 to 420;ERL-4206 has a freezing point of-55° C., a specific gravity of 1.09, anepoxy equivalent weight of from about 70 to about 74 and an averageapproximate molecular weight of from about 140 to 148; ERL4234 has amelting point of approximately 0° C., a specific gravity of 1.18, anepoxy equivalent weight of from about 133 to about 154 and an averageapproximate molecular weight of from about 266 to 318.

A suitable phenoxy polyol resin is available under the trade name“PKHC”, or “PKHJ”, formerly from Union Carbide Corporation, now fromPhenoxy Resin Associates. A suitable octafunctional bisphenol A,formerly available under the trade name “Epirez SU8” from High TekPolymers, is now available as “Epon SU8” from Shell Chemical Company. Asuitable tetrabromobisphenol A formerly available under the trade name“Epirez 5183” from High Tek Polymers, is now available as “Epon 1183”from Shell Chemical Company. A suitable complex triarylsulfoniumhexafluoroantimonate salt photoinitiator, formerly available under thetrade name UVE 1014 from General Electric Company, is now available asUVI 6974 from Union Carbide Company. The UVI 6974 is a 50% solution oftriarylsulfonium hexafluoroantimonate salt in propylene carbonate.

The solids of the photoimageable dielectric film optionally comprise aparticulate rheology modifier, preferably a thixotropic particulaterheology modifier. Preferably, the particulate rheology modifier has anaverage particle size of from about 0.001 to about 10 microns, morepreferably from about 0.01 to about 5 microns. Examples of particulaterheology modifiers are barium sulfate, talc, aluminum oxide, antimonyoxide, kaolin, finely divided silicon dioxide which may be colloidal orrendered hydrophobic, micronised talcum, micronised mica, kaolin,aluminum oxide, aluminum hydroxide, calcium silicate, aluminum silicate,magnesium carbonate, calcium carbonate, zirconium silicate, porcelainpowder, glass powder, antimony trioxide, titanium dioxide, bariumtitanate and barium sulfate or mixtures thereof

Preferably, the particulate rheology modifier is silica; a suitablesilica is available under the trade name Aerosil A380 silica fromDegussa. Where the particulate rheology modifier is a thixotropicparticulate rheology modifier, it is preferred that no more than 30% beused.

Optionally, a surfactant, such as for example a nonionic surfactant, isemployed. Preferably the surfactant is a fluoronated polyether; asuitable surfactant is available under the trade name FC-430, from 3MCompany.

Any residual solvent content left over from the manufacturing process inthe uncured photoimageable dielectric film preferably ranges from about0 to 5%, more preferably from about 0 to 3%, most preferably from about0 to less than about 2%, by weight of the film.

While the invention had been described with a certain degree ofparticularity, various adaptations and modifications can be made withoutdeparting from the scope of the invention as defined in the appendedclaims.

What is claimed is:
 1. A process for manufacturing an interconnectstructure containing a build-up dielectric layer containing at least oneopening comprising the steps of: a) providing a substrate, b) coatingthe substrate with a dielectric composition for making interconnectfilms, said composition comprising: 1) an epoxy base resin consistsessentially of: (i) between about 10% and about 80% by weight of apolyol resin which is a condensation product of epichlorohydrin andbisphenol A; (ii) about 0% to about 80% by weight of a multifunctionalbisphenol A formaldehyde novalak resin; (iii) about 10% to about 50% byweight of a glycidyl ether of bisphenol A; and (iiii) about 10% to about35% of a liquid epoxy resin 2) from about 0.1 to about 15 parts byweight per 100 parts of a curing agent capable of initiatingpolymerization of said epoxy base resin; 3) from about 0.01 to about 1parts by weight of a dye having a substantial absorbtivity at thewavelength of emission of the light of a laser beam; c) curing the film;and d) forming at least one opening in the dielectric composition bylaser ablation.
 2. The invention of claim 1 wherein the curing agent isa thermally activated compound which can initiate polymerization of theepoxy functionality by exposure to temperatures above about 125° C. 3.The invention of claim 1 wherein the curing agent is a thermally labilecationic photoinitiator which can initiate polymerization of the epoxyfunctionality by exposure to light or by exposure to temperatures aboveabout 125° C. in the absence of light.
 4. The invention of claim 1wherein the dye is selected from the group consisting of rhodamine B,ethyl violet, 2′, 7′-dichlorofluorescein, carbostyril 124, carbostyril165, coumarin 2, coumarin 120, coumarin 4, p-terphenyl, and fluorescein.